Recommendations for more effective long-term weight loss strategies may need to consider the role of gender differences. If, as shown in female vs. male animal models, negative energy balance resulting in weight loss results in greater compensatory reductions in energy expenditure in women compared to men, obesity treatments may need to be tailored in women to override these reductions in total energy expenditure. Our approach focuses on a behavioral strategy (self-monitoring) to eliminate the compensatory reduction in non-exercise "spontaneous" physical activity (SPA) seen in women who lose weight by means of a hypocaloric diet and structured exercise training. Our long-term research goal is to establish empirical evidence for innovative treatment options that are more effective in producing weight loss and preventing weight regain in women. The main goal of this pilot is to provide preliminary data and effect estimates to begin to test our overall hypothesis that prevention of weight loss-induced reductions in SPA will be more beneficial for long-term maintenance of weight loss in women than in men. We propose to conduct a pilot study using a 2-arm, 10-month design in 72 obese, older (55-70 yrs) men and women (n=36 per group). Participants will be randomized to a 5-month standardized weight loss intervention involving a hypocaloric diet and aerobic exercise (DIET+EX) or to the same weight loss intervention with addition of a behavioral component that targets self-monitoring (SM) of SPA (SM+DIET+EX), and then followed for another 5 months after weight loss. The specific aims of this R21 exploratory/developmental application are: Primary-To examine whether SPA self- monitoring results in less body weight regain in the follow-up phase in both men and women;Secondary-To examine whether: 1) women regain more weight than men in the follow-up phase;2) SPA self-monitoring and gender have an effect on change in weight in the intensive weight loss phase;3) SPA self-monitoring and gender have an effect on change in SPA in the intensive weight loss phase;4) there is an association between SPA changes in the weight loss phase and weight regain in the follow-up phase. We anticipate that the results will lead to a larger and longer trial to definitively test our hypothesis, which could potentially provide evidence against the current standard of care (i.e., exclusive prescription of structured moderate-intensity exercise) for obesity therapy in women and may lead to sex-specific treatment guidelines. PUBLIC HEALTH RELEVANCE: Weight loss programs using caloric restriction and regular, structured exercise can lead to a reduction in physical activity performed outside of the planned exercise sessions. This study will test whether or not women reduce their spontaneous daily physical activity more than men with weight loss, and whether this reduction can be prevented by using self-monitoring, thereby slowing or preventing weight regain. Since reductions in spontaneous physical activity could potentially negate the intent of a structured exercise program for weight loss therapy, it would be important to know whether recommendations for weight loss maintenance in women should include promotion of spontaneous physical activity, rather than structured exercise, during and following a period of intensive weight loss.